"Hallelujah!"
How did you just read that word? Were you singing it with the strains of "Handel's Messiah" ringing in your head? Or was it with a frustrated tone, as in "Finally!"? Or maybe you have just woken up and are still getting through your first cup of coffee, and so it was merely a flat monotone!
I put a lot of expression into my writing. Lately I have been feeling very lighthearted and on top of the world, and so my tone in my letters has been very chirpy. However, it suddenly occurred to me that if the recipient were not feeling so chirpy, in fact maybe they are downright sullen and serious, then they would read my writing in a totally different tone of voice! And how that would change the way they heard it.
It reminds me of this great quote: "I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant." So what counts is not actually the way you or I wrote something, but actually the way the other person read it. And the frustrating thing is - we have no control over that!